Perseus is a constellation in the northern sky, being named after the Greek mythological hero Perseus. It is one of the 48 listed by the 2nd-century astronomer Ptolemy, and among the 88 modern constellations defined by the International Astronomical Union. It is located near several other constellations named after ancient Greek legends surrounding Perseus, including Andromeda to the west and Cassiopeia to the north. Perseus is also bordered by Aries and Taurus to the south, Auriga to the east, Camelopardalis to the north, and Triangulum to the west. Some star atlases during the early 19th century also depicted Perseus holding the disembodied head of Medusa, whose asterism was named together as Perseus et Caput Medusae, however, this never came into popular usage. Covering 615 square degrees, it ranks twenty-fourth of the 88 constellations in size. It appears prominently in the northern sky during the Northern Hemisphere’s spring. Its main asterism consists of 19 stars. The constellation’s boundaries, as set by Eugène Delporte in 1930, are defined by a 26-sided polygon. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 01h 29.1m and 04h 51.2m, while the declination coordinates are between 30.92° and 59.11°.
Algol (from the Arabic The Demon’s Head), also known by its Bayer designation Beta Persei, is the best-known star in Perseus. Representing the head of the Gorgon Medusa in Greek mythology. Located 92.8 light-years from Earth, it varies in apparent magnitude from a minimum of 3.5 to a maximum of 2.3 over a period of 2.867 days. The star system is the prototype of a group of eclipsing binary stars named Algol variables, though it has a third member to make up what is actually a triple star system. The brightest component is a blue-white main-sequence star of spectral type B8V, which is 3.5 times as massive and 180 times as luminous as the Sun. The secondary component is an orange subgiant star of type K0IV that has begun cooling and expanding to 3.5 times the radius of the Sun and has 4.5 times the luminosity and 80% of its mass. These two are separated by only 0.05 astronomical units (AU); the main dip in brightness arises when the larger fainter companion passes in front of the hotter brighter primary. The tertiary component is a main sequence star of type A7, which is located on average 2.69 AU from the other two stars. AG Persei is another Algol variable in Perseus, whose primary component is a B-type main sequence star with an apparent magnitude of 6.69. Phi Persei is a double star, although the two components do not eclipse each other. The primary star is a Be star of spectral type B0.5, possibly a giant star, and the secondary companion is likely a stellar remnant. The secondary has a similar spectral type to O-type subdwarfs. With the historical name Mirfak (Arabic for elbow) or Algenib, Alpha Persei is the brightest star of this constellation with an apparent magnitude of 1.79. A supergiant of spectral type F5Ib located around 590 light-years away from Earth, Mirfak has 5,000 times the luminosity and 42 times the diameter of our Sun. It is the brightest member of the Alpha Persei Cluster (also known as Melotte 20 and Collinder 39), which is an open cluster containing many luminous stars. Zeta Persei is the third-brightest star in the constellation at magnitude 2.86. Around 750 light-years from Earth, it is a blue-white supergiant 26–27 times the radius of the Sun and 47,000 times its luminosity. It is the brightest star (as seen from Earth) of a moving group of bright blue-white giant and supergiant stars, the Perseus OB2 Association or Zeta Persei Association. Zeta is a triple star system, with a companion blue-white main sequence star of spectral type B8 and apparent magnitude 9.16 around 3900 AU distant from the primary, and a white main sequence star of magnitude 9.90 and spectral type A2, some 50,000 AU away, that may or may not be gravitationally bound to the other two.
The galactic plane of the Milky Way passes through Perseus but is much less obvious than elsewhere in the sky as it is mostly obscured by molecular clouds. The Perseus Arm is a spiral arm of the Milky Way galaxy and stretches across the sky from the constellation Cassiopeia through Perseus and Auriga to Gemini and Monoceros. This segment is towards the rim of the galaxy. Within the Perseus Arm lie two open clusters (NGC 869 and NGC 884) known as the Double Cluster. Sometimes known as h and Chi (χ) Persei respectively Both lie more than 7,000 light-years from Earth and are several hundred light-years apart. Both clusters are of approximately magnitude 4 and 0.5 degrees in diameter. The two are Trumpler class I 3 r clusters, though NGC 869 is a Shapley class f and NGC 884 is a Shapley class e cluster. These classifications indicate that they are both quite rich (dense); NGC 869 is the richer of the pair. The clusters are both distinct from the surrounding star field and are clearly concentrated at their centres. The constituent stars, numbering over 100 in each cluster, range widely in brightness. M34 is an open cluster that appears at magnitude 5.5 and is approximately 1,500 light-years from Earth. It contains about 100 stars scattered over a field of view. IC 348 is a young open cluster that is still contained within the nebula from which its stars formed. It is located about 1,027 light-years from Earth, is about 2 million years old, and contains many stars with circumstellar disks. Many brown dwarfs have been discovered in this cluster due to its age; since brown dwarfs cool as they age, it is easier to find them in younger clusters. There are many nebulae in Perseus. M76 is a planetary nebula, also called the Little Dumbbell Nebula. It appears two arc-minutes by one arc-minute across and has an apparent brightness of magnitude 10.1. NGC 1499, also known as the California Nebula, is an emission nebula that was discovered in 1884–85 by American astronomer Edward E. Barnard. NGC 1333 is a reflection nebula and a star-forming region. Perseus also contains a giant molecular cloud, called the Perseus molecular cloud; it belongs to the Orion Spur and is known for its low rate of star formation compared to similar clouds. Perseus contains some notable galaxies. NGC 1023 is a barred spiral galaxy of magnitude 10.35, around 11.6 million pc (38 million ly) from Earth. It is the principal member of the NGC 1023 group of galaxies and is possibly interacting with another galaxy. NGC 1260 is either a lenticular or tightly-wound spiral galaxy about 76.7 million pc (250 million ly) from Earth. It was the host galaxy of the supernova SN 2006gy, one of the brightest ever recorded. It is a member of the Perseus cluster (Abell 426), a massive galaxy cluster located 76.6 million pc (250 million ly) from Earth. With a redshift of 0.0179, Abell 426 is the closest major cluster to the Earth. NGC 1275, a component of the cluster, is a Seyfert galaxy containing an active nucleus that produces jets of material, surrounding the galaxy with massive bubbles. These bubbles create sound waves that travel through the Perseus Cluster. This galaxy is a cD galaxy that has undergone many galactic mergers throughout its existence, as evidenced by the “high-velocity system”—the remnants of a smaller galaxy—surrounding it. Its active nucleus is a strong source of radio waves.
The Perseids are a prominent annual meteor shower that appears to radiate from Perseus from mid-July, peaking in activity between 9 and 14 August each year. Associated with Comet Swift–Tuttle, they have been observed for about 2,000 years. The September Epsilon Perseids, discovered in 2012, are a meteor shower with an unknown parent body in the Oort cloud. Credit: Wikipedia.